The present invention relates generally to the packaging of electronic components. More particularly, the present invention relates to a method of fabricating an image sensor package.
Image sensors and assemblies are well known to those of skill in the art. In these assemblies, an image sensor was located within a housing, which supported a window. Radiation passed through the window and struck the image sensor, which responded to the radiation.
In the assembly, an image sensor was mounted to a printed circuit board. After the image sensor was mounted, the housing was mounted around the image sensor and to the printed circuit board. This housing provided a hermetic like barrier around the image sensor, while at the same time, supported the window above the image sensor.
As the art moves to smaller and lighter weight electronic devices, it becomes increasingly important that the size of the image sensor assembly used within these electronic devices is small. Disadvantageously, conventional image sensor assemblies required a housing to support the window and to hermetically seal the image sensor. However, these housings were relatively bulky and extend upwards from the printed circuit board a significant distance, e.g., 0.100 inches (2.54 mm) to 0.120 inches (3.05 mm) or more.
In addition, mounting these housings at the printed circuit board level was inherently labor intensive and made repair or replacement of the image sensor difficult. In particular, removal of the housing exposed the image sensor to the ambient environment. Since the image sensor was sensitive to dust as well as other environmental factors, it was important to make repairs or replacement of the image sensor in a controlled environment such as a cleanroom. Otherwise, there was a risk of damaging or destroying the image sensor. Since neither of these alternatives are desirable and both are expensive, the art needs an image sensor assembly which is simple to manufacture and service so that costs associated with the image sensor assembly are minimized.
In the event that moisture was trapped inside of the housing, defective operation or failure of the image sensor assembly was observed. More particularly, the moisture had a tendency to condense within the housing and on the interior surface of the window. Even if the housing later dried out, a stain was left on the window. In either event, electromagnetic radiation passing through the window was distorted or obstructed by either moisture condensation or stain, which resulted in defective operation or failure of the image sensor assembly.
For this reason, an important characteristic was the temperature at which condensation formed within the housing of image sensor assembly, i.e., the dew point of the image sensor assembly. In particular, it was important to have a low dew point to insure satisfactory performance of the image sensor assembly over a broad range of temperatures.
In accordance with the present invention, an image sensor package includes an image sensor having an active area, which is responsive to radiation. The image sensor is mounted to a substrate, which is transparent to the radiation. The image sensor is mounted such that the active area of the image sensor faces the substrate.
During use, radiation is directed at the substrate. This radiation passes through the substrate and strikes the active area of the image sensor. The image sensor responds to the radiation in a conventional manner.
Of importance, the substrate serves a dual function. In particular, the substrate is the window which covers the active area of the image sensor. Further, the substrate is the platform upon which the image sensor package is fabricated.
Recall that in the prior art, a housing was used to support the window above the image sensor. These housings were typically formed of ceramic, which is relatively expensive. Advantageously, an image sensor package in accordance with the present invention eliminates the need for a housing of the prior art. Accordingly, the image sensor package is significantly less expensive to manufacture than image sensor assemblies of the prior art.
In one embodiment, a bead is formed around a periphery of the image sensor such that the image sensor, the bead, and the substrate form a sealed cavity. The active area of the image sensor is located and hermetically sealed within this cavity. Hermetically sealing the active area reduces complexity and cost in the event the image sensor must be repaired or replaced compared to the prior art.
Recall that in the prior art, the housing, which hermetically sealed the image sensor, was mounted directly to the larger substrate. Thus, removal of the housing necessarily exposed the image sensor to the ambient environment and to dust. For this reason, the image sensor had to be repaired or replaced in a cleanroom or else there was a risk of damaging or destroying the image sensor.
In contrast, the active area is hermetically sealed as part of the image sensor package. The image sensor package is mounted to the larger substrate. To repair or replace the image sensor, the image sensor package is simply removed and a new image sensor package is mounted to the larger substrate. At no time is the active area exposed to the ambient environment during this procedure. Advantageously, this procedure can be performed in any facility with or without a cleanroom. The old image sensor package is discarded or shipped to a central facility for repair. Since the image sensor package is simple to manufacture and service, the cost associated with the image sensor package are minimized compared to the prior art.
Further, the image sensor package is relatively thin compared to prior art image sensor assemblies. In particular, by mounting the image sensor directly to the substrate, which also serves as the window for the image sensor, the resulting thickness of the image sensor package is relatively small, e.g., is 0.99 millimeters (mm). Recall that in the prior art, the image sensor was mounted directly to the larger substrate and a housing was used to support a window above the image sensor. This housing extended a significant distance, e.g., 0.100 inches (2.54 mm) to 0.120 inches (3.05 mm) or more, from the larger substrate. Since the image sensor package in accordance with the present invention is relatively thin compared to an image sensor assembly of the prior art, the image sensor package is well suited for use with miniature lightweight electronic devices, which require thin and lightweight image sensor assemblies.
In another embodiment, a step up ring is used to elevate interconnection balls above the image sensor, the interconnection balls being used to connect the image sensor package to a larger substrate such as a printed circuit mother board. Advantageously, use of the step up ring allows the interconnection balls to have minimum size and pitch. This may be important, for example, when a large number of interconnection balls must be provided in a limited area.
In one embodiment, a plurality of image sensor packages are fabricated simultaneously to minimize the cost associated with each individual image sensor package. In accordance with this embodiment, image sensors are attached to an array type substrate, which includes a plurality of individual substrates integrally connected together. Beads are formed around the peripheries of the image sensors. The array type substrate is then singulated, either before or after the array type substrate is populated with interconnection balls or other interconnection structures.
By forming a plurality of image sensor packages simultaneously, several advantages are realized. One advantage is that it is less labor intensive to handle and process a plurality of image sensor packages simultaneously rather than to handle and process each image sensor package on an individual basis. By reducing labor, the cost associated with each package is minimized.
In accordance with another embodiment of the present invention, an image sensor package includes a substrate having an aperture. The aperture is defined by an aperture side. The image sensor package further includes an image sensor having an active area aligned with the aperture. A window is in contact with the aperture side. In one embodiment, the window is formed of a hardened transparent liquid encapsulant.
Advantageously, by forming the window with a low refractive index, the sensitivity of the image sensor package is improved compared to the prior art. Recall that in the prior art, a housing was mounted around the image sensor and to the print circuit board. This housing supported a window above the image sensor. However, located between the window and the image sensor was air. Disadvantageously, air has a relatively low refractive index compared to the window. As those skilled in the art understand, as visible light or other electromagnetic radiation passes from a material having a high refractive index to a material having a low refractive index and vice versa, a significant percentage of the electromagnetic radiation is reflected.
Since the electromagnetic radiation had to pass from air, through the window, and back through air to reach the active area of the image sensor in the prior art, a significant percentage of the electromagnetic radiation was reflected. In particular, the electromagnetic radiation had to pass through three interfaces: (1) the air/window interface; (2) the window/air interface; and (3) the air/active area interface. This resulted in an overall loss of sensitivity of prior art image sensor assemblies.
However, in the image sensor package in accordance with this embodiment, radiation passes from air, through the window, and reaches the active area. Accordingly, the radiation passes through only two interfaces: (1) the air/window interface; and (2) the window/active area interface. By minimizing the number of interfaces, the amount of reflected radiation is also minimized. Accordingly, the amount of reflected radiation is reduced compared to the prior art. This improves the sensitivity of the image sensor package compared to prior art image sensor assemblies.
Further, instead of having air between the window and the active area of the image sensor as in the prior art, the window completely fills the region between the ambient environment and the active area. Advantageously, by eliminating the prior art cavity between the active area and the window, the possibility of moisture condensation within the cavity is also eliminated. Accordingly, the image sensor package does not have a dew point.
In contrast, prior art image sensor assemblies had a dew point, i.e., a temperature at which condensation formed within the housing, which enclosed the image sensor and supported the window. Disadvantageously, this limited the temperature range over which the image sensor assembly would satisfactorily perform. Alternatively, the image sensor assembly was fabricated in a low humidity environment to avoid trapping moisture within the housing and was hermetically sealed by the housing to keep out moisture. This added complexity, which increased the cost of the image sensor assembly. Further, in the event that the hermetic seal of the housing failed, the image sensor was damaged or destroyed.
Since the image sensor package in accordance with this embodiment does not have a dew point, the image sensor package operates satisfactorily over a broader range of temperatures than image sensor assemblies of the prior art. Further, since the image sensor package is formed without a cavity, there is no possibility that moisture will leak into the image sensor package. Accordingly, the reliability of the image sensor package is greater than that of the prior art.
The window also relieves stress on the bumps between the bond pads of the image sensor and the traces on the substrate. In particular, to the extent that the image sensor has a different thermal coefficient expansion than the substrate, the window insures that the image sensor does not become dismounted from the substrate as a result of differential thermal expansion. By minimizing the possibility of failure of the bumps, the window insures the reliability of the image sensor package.
Advantageously, by mounting the image sensor to the substrate as a flip chip, the image sensor is positionally aligned to within tight tolerances. More particularly, since the bond pads of the image sensor are connected to the traces on the substrate, the image sensor is inherently aligned to the traces. Further, since the interconnection balls are formed on these same traces, the interconnection balls are inherently aligned to the traces. As a result, the image sensor is precisely aligned to the interconnection balls. By precisely aligning the image sensor, the performance of the image sensor package is improved compared to a conventional image sensor assembly in which bond pads were wirebonded to traces.